High-temperature isostatic pressing of articles



Jan. 9, 1968 R. P. LEVEY, JR, ETAL 3,363,0

HIGH-TEMPERATURE ISOSTATIC PRESSING OF ARTICLES Filed July 1, 1965 2 Sheets-Sheet 1 FIG! INVENTORS. Ralph P. Levey, Jr. Alfred E. Smifh M Q U ATTORNEY.

Jan. 9, 1968 R. P. LEVEY, JR., ETAL 3,363,037

HIGH-TEMPERATURE ISOSTATIC PRESSING OF ARTICLES Filed July 1, 1965 2 Sheets-Sheet 2 INVENTORS. Ralph P Levey, Jr.

Alfred E. Smifh ATTORNEY "United States Patent Ofiiice 3,363,037 Patented Jan. 9, 1968 3,363,037 HIGH-TEMPERATURE ISOSTATHZ PRESSING OF ARTICLES Ralph P. Levey, in, Oak Ridge, and Alfred E. Smith, South Clinton, Tenn., assignors to the United States of Americe as represented by the United States Atomic Energy Commission Filed July 1, 1965, Ser. No. 469,009 3 Claims. (Cl. 264-125) ABSTRACT OF THE DISCLOSURE A method of isostatically pressing a body formed from a powder comprising embedding the body together with a resistance heating element within a mass of sand enclosed within a flexible walled container and then applying heat to the body and pressure to the walls of the container until the powder particles making up the body are sintered and compacted.

Our invention relates to methods of making articles from powders.

As is well known, articles made from powders by hotpressing are superior in many properties to articles made by cold-pressing, and the only method of forming nonporous compacts of precise dimensions from many powders is by hot-pressing. However, industrial processes for hot-pressing are limited to forming articles having relatively simple shapes. As the shapes become complex, undesirable density gradients are produced in the finished product because of non-uniform pressure distribution during pressing. Industrial processes are additionally limited to use of relatively low pressures, i.e., less than 5,000 pounds per square inch, and to temperatures no greater than about 1400 C.

One method of forming articles from powders, isostatic pressing, avoids the disadvantages resulting from non-uniform distribution of pressure by transmitting pressure from the press to surfaces of the article by means of a fluid in pressure-transmitting relationship with these surfaces. However, high-temperature isostatic pressing has not been economical because of the expensive apparatus needed to contain the rlluid, a gas, at an elevated temperature and pressure. The temperatures and pressures employed in prior methods of isostatic pressing are limited by the strength of the construction materials at elevated temperature since design of the equipment requires that a substantial portion of the apparatus be at an elevated temperature.

It is one object of our invention to provide an improved hot isostatic pressing method.

It is another object to provide a hot isostatic pressing method which does not subject the structural portion of the pressing apparatus to a high temperature.

It is still another object to provide a method for isostatically pressing relatively large articles .at extremely high temperatures and pressures.

Other objects of our invention will be apparent from the following description and appended claims.

In accordance with our invention we have provided a method of isostatically pressing a body at an elevated temperature comprising the steps of: disposing said body in heat-exchange relationship with a heating element; embedding the body and its associated heating element within a mass of sand contained within a flexible container; raising the temperature of said heating element whereby said body is raised to an elevated temperature; and applying pressure to the walls of said container.

Our apparatus is so designed that all structural elements are at a temperature far lower than the temperature of the article to be pressed, thus permitting the article to be heated to a temperature much higher than is possible using other configurations. The combination of (1) large size of object which can be pressed, (2) isostatic pressing conditions, and (3) the extremely high temperatures and pressures which can be employed has not been achieved by other methods. The materials used such as the heater and the container are inexpensive and expendable.

FIG. 1 shows one adaptation of our invention. In the figure an article to be pressed 1 and an encircling resistance coil 2 are embedded within an insulating, pressuretransmitting sand 3 contained within a flexible container 4. Thermocouples 5, 6, 7, and 8 are also embedded Within the sand to measure the temperature at various zones within the body of sand. Evacuation tube 9 is provided to remove gases from the zone enclosed by container 4, and electrical leads 10 and 11 are in electrical communication with resistance coil 2.

FIG. 2 shows the assembly 12 of FIG. 1 immersed in oil 13 contained within a pressure vessel comprising a hollow cylindrical body 14 and cylindrical plungers 15 and 16.

In pressing an article in accordance with our method the article to be pressed is first formed into a shape and is then embedded within the sand close to the coils of the resistance heater. Pressure is then applied to the readily deformable container while the article is being heated by the resistance coil.

Our method may be used to press any powder capable of being formed into a coherent body by pressing, and is especially useful in sintering under pressure refractory metals and metal oxides.

The size of the object to be pressed is limited only by the size of the equipment employed to apply pressure to the flexible container. Bars up to 12 inches in diameter have been pressed at high temperatures and pressures using our method.

The sand selected must be (1) heat resistant, (2) noncompacting (its grain strength must exceed the pressure employed), and (3) inert to the powder being pressed at the temperature and pressure used. Typically useful sands for pressing refractory metals are zirconium oxide, uranium dioxide, and aluminum oxide. Magnesium oxide transmits pressure for efficiently than other sands and is therefore preferred. Measurements made of pressures near the powder compact indicate that with magnesium oxide the pressure experienced by the specimen is completely isostatic, and unexpectedly is slightly higher (about 1-5 percent) than the pressure exerted on the container. The size of the particles of sand may range from 4 to mesh (U.S. sieve series) and are preferably from 4 to 20 mesh.

The heating element should be as close to the powder compact as possible in order to heat the body efficiently to the desired temperature.

Inasmuch as the sand has excellent insulating properties, the temperature of the powder compact can be raised to a high level and maintained there for a substantial period of time. Factors such as rate of power input, the spacing between the powder compact and the heating element, and the distance between the heating element and the container wall determine the maximum temperature obtainable. With a tungsten filament temperatures for a powder compact of about 2600 C. have been maintained for about 2 hours.

The container may be made of any flexible material inert to the sand and capable of withstanding the temperature anticipated at the container wall. Organic materials as exemplified by the polymerized diolefins are suitable in processes where the heat input is low. The

3 preferred materials are metals as exemplified by copper, lead, aluminum, and iron-base alloys.

Having thus described our invention the following example is offered to illustrate it in more detail.

EXAMPLE Tungsten powder having an average particle diameter of 4 microns was isostatically cold-pressed at a pressure of 30,000 pounds per square inch to form a coherent mass 1 /2 inches in a diameter and 2 /2 inches long. A tungsten filament was coiled about the compact and embedded in MgO sand (4-8 mesh) held in a lS-mil copper bag 16 inches in diameter and 36 inches long. The bag and its contents were immersed in the oil-filled chamber of a pressure vessel. A pressure of 15,000 pounds per square inch was exerted on the exterior of the container while electrical power kilowatts) was applied to the resistance heater. The compact attained a temperature of about 2600 C. which was maintained for about 2 hours. The temperature near the wall of the container was about 150 C. A completely dense isotropic finegrained body was produced.

The above example is merely illustrative and it is obvious that changes may be made without departing from our invention.

Having thus described our invention, we claim:

1. A method of hot pressing a body formed of powder material comprising the steps of:

(a) placing said body in a heat-exchange relationship to an electrical resistance heating element;

(b) embedding said body and associated heating element within a mass of sand filling a closed container, said sand comprising particles of 4 to 100 mesh size, said closed container having relatively flexible walls;

(c) placing said container within an isostatic pressure means;

(d) applying an electric current to said resistance heating element to heat said body to a temperature at which said powder of said body sinters;

(e) isostatically applying sufiicient pressure inwardly against the walls of said container to press the heated body into a compact; and

(f) removing said pressed body.

2. The method of claim 1 wherein said sand is MgO.

3. The method of claim 1 wherein said sand is MgO in the size range of 4 to 20 mesh.

References Cited UNITED STATES PATENTS 2,990,602 7/1961 Brandmayr et a1 264- 

